Compositions, based on ethylene polymers such as ethylene-vinyl acetate copolymers and ethylene-ethyl acrylate copolymers, by reason of their advantageous properties, for example, corrosion resistance, toughness and abrasion resistance, have been widely used in molding and extrusion applications. Ethylene polymers, however, being flammable, require the addition of flame retardant additives in order that compositions based on these polymers be acceptable for use in the applications indicated. Illustrative of additives which have been used for this purpose are antimony oxide and halogenated flame retardant additives.
Compositions based on ethylene polymers, containing antimony oxide or a halogenated flame retardant additive, suffer the disadvantages, on burning, of producing dense smoke and/or evolving high levels of noxious and corrosive gases. Consequently, other additives such as hydrated inorganic compounds, exemplified by aluminum hydroxide and magnesium hydroxide have been added to ethylene polymer compositions for the purpose of improving the flame retardant properties thereof as disclosed in Japanese Published Patent Nos. 45-263 and 48-18570.
The presence of hydrated inorganic compounds in ethylene polymer compositions presents a number of advantages. When subjected to heat, these hydrated inorganic compounds undergo an endothermic reaction and liberate water. The water, in turn, decreases the flammability of the compositions, mitigates against the spread of flames and minimizes the emission of smoke. Furthermore, smoke and gases emitted by the described ethylene polymer compositions, containing hydrated inorganic compounds, are non-toxic. In addition, such compositions do not drip burning material.
A drawback, however, with respect to hydrated inorganic compounds is the large quantity that has to be used in order to impart a high degree of flame retardancy to ethylene polymer compositions. The addition of a large quantity of hydrated inorganic compounds decreases the mechanical strengths, particularly impact resistance, of the resultant compositions and impairs the flow properties thereof, represented by melt index, leading to poor moldability.
In order to remedy these drawbacks, alumina trihydrate, treated with a silicone coupling agent, has been proposed as a flame retardant additive to ethylene polymer compositions, as disclosed in Japanese Laid-Open Patent No. 56-20040. The technology of this patent is disadvantageous in that it is difficult to intimately mix the treated alumina trihydrate with the polymer matrix. Consequently, the resultant composition is unsatisfactory from the standpoint of mechanical and flow properties.